It says in section "3. wsrep Settings" that you can start the first cluster node with:
service mysql start --wsrep-cluster-address=gcomm://
But it does not work. The wsrep-cluster-address is still taken from my.cnf.
Giving it on command line should be good since you do not have to edit my.cnf, and then risk not editing it back to connect to a running cluster (since you don't want to start a second node as master).